Cisco 2950 Qos question

[per.nielsen]

On the Cisco 2950 Switch a default CoS-to-DSCP mapping table exists.

How come that 802.1p Cos=5 get's mapped to DSCP=40.

Normally Cos=5 should be mapped to EF DSCP=46

Can anyone explain that wierd mapping ??

Cheers

Per

[pkhatri · Accepted solution]

Hi Per,

I believe it is a hardware limitation of the 2950 switch which only allows it to support a small subset of the total number of DSCP values. It is an annoyance and something that needs to be kept in mind when designing end-to-end QoS with 2950 switches in the middle.

The default mapping is a simple multiplication of the CoS value by 8. That's why CoS 5 gives you 40. In DSCP terminology, this would be Class Selector 5 or cs5.

Unfortunately, there is no way to change this to DSCP 46 on the 2950s....

Hope that helps - pls do remember to rate posts that help.

Paresh

[bbaillie · Accepted solution]

The default mapping is what it is in the standard image 2950 switches, unfortunately. The 2950 switches that run "enhanced image" allow the changing of the DSCP-COS and COS-DSCP mappings if you desire the functionality. The details are in a FAQ document.

http://www.cisco.com/en/US/tech/tk389/tk813/technologies_q_and_a_item09186a00804976d3.shtml

Document ID: 46523

Brian